Duration of Pregnancy

Duration of Pregnancy

When will the baby be born? The average length of time from conception to birth is 266 days. However, very rarely is the exact date of conception known, so the convention is to calculate the expected date of delivery (EDD) by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). This method of calculation assumes that conception took place two weeks after the start of the last period, when ovulation is most likely. Not all women ovulate at precisely this stage in the menstrual cycle, so it is only an estimate. An ultrasonic scan may be useful in cases of uncertainty about the age of the unborn baby. This technique can give fairly exact information on the duration of the pregnancy by measuring the diameter of the embryo’s head.

Even so, rather than gearing all expectations to giving birth on an expected date of delivery, it is better to think of it only as a guide to when a woman is likely to deliver. There is an 85 per cent chance that the baby will be born at any time from two weeks before to two weeks after the EDD. If someone is expecting twins, pregnancy is usually shorter – about 37 weeks rather than the normal 40.

False pregnancy

A woman can occasionally be convinced that she is pregnant when she is not. She may even have most of the symptoms of pregnancy, such as nausea, breast changes, missed periods and increase in weight and, if a pregnancy test is not performed, the diagnosis of pregnancy may be accepted on the grounds of these symptoms alone. Eventually, because the uterus does not increase in size, it will be evident to her doctor that the woman is not pregnant, but she may find it very difficult to accept the truth.

Such a false pregnancy (pseudocyesis) may follow sterilization, miscarriage or stillbirth, or may occur in a woman approaching the menopause who has been trying unsuccessfully for years to conceive. Although this suggests psychological causes, in some cases there is evidence that a hormonal derangement of the ovaries may be causing the physical changes associated with pseudocyesis. Thus it is not ‘all in the mind’.